1. Field of the Invention
The present invention relates to a process for the catalytic oxidation of liquid cycloparaffins, especially cyclohexane, in which the oxidation catalyst is a combination of heavy metal compounds. The present invention also relates to an improved process for the manufacture of dicarboxylic acids, especially adipic acid.
2. Description of the Prior Art
The oxidation of cycloparaffins to produce useful partial oxidation products, for example, the oxidation of cyclohexane to cyclohexanol and cyclohexanone, is known as one important step in the manufacture of nylon intermediates, for example, adipic acid. In the case of adipic acid manufacture, it has been found to be preferable to oxidize cyclohexane to adipic acid in a two-step oxidation process, i.e., to oxidize cyclohexane to a mixture containing cyclohexanol and cyclohexanone and to thereafter oxidize that mixture to adipic acid using nitric acid, for example, as described by C. H. Hamblet and A. McAlevy in U.S. Pat. No. 2,557,282, which issued June 19, 1951. Copper and/or vanadium catalysts may be used in the nitric acid oxidation step.
A process in which cyclohexane is oxidized in the liquid phase to cyclohexanol and cyclohexanone at low conversion and high yields was disclosed by D. J. Loder in U.S. Pat. No. 2,223,494, which issued Dec. 3, 1940. In the Loder process, the preferred catalysts include cobalt alkanoate, especially cobalt naphthenate. The yields of cyclohexanol and cyclohexanone obtained by the Loder process were considerably higher than had been achieved in earlier processes not employing catalysts or initiators. While Loder's process when operated on a noncommerical scale, can be made to give yields of cyclohexanol plus cyclohexanone from cyclohexane of 85-95%, in practical commercial operations it is often necessary to compromise on yield in favor of other process parameters in order to have a continuous process which can be run in an economical manner.
It is known that the quantities of useful oxidation products and the by-products of the oxidation of cycloparaffins depend on, in particular, the temperature, residence time, oxygen concentration and flow rates of cycloparaffin, and the like. An improvement in the method for the oxidation of cyclohexane and for the control of the oxidation products so obtained is disclosed by K. Pugi in U.S. Pat. No. 3,530,185, which issued Sept. 22, 1970.
A process for the catalytic oxidation of cycloparaffins to the corresponding cycloalkanol and cycloalkanone in the presence of a catalyst that is, for example, a cobalt monoalkylphosphate and/or cobalt dialkylphosphate, is disclosed by A. Kuessner et al. in U.S. Pat. No. 3,917,708, which issued Nov. 4, 1975. R. A. Zelonka, in U.S. patent application No. 229,873, filed Jan. 30, 1981, now U.S. Pat. No. 4,341,907 discloses a process for the catalytic oxidation of cycloparaffins to the corresponding cycloalkanol and cycloalkanone in the presence of a catalyst that is a cobalt compound in combination with a heterocyclic compound. A process for the preparation of cyclohexanone and cyclohexanol in which cyclohexane is catalytically oxidized in the presence of a binary cobalt/chromium catalyst and subsequently treated to decompose cyclohexylhydroperoxide is disclosed by W. J. Barnette et al. in U.S. Pat. No. 3,987,100, which issued Oct. 19, 1976.
Commercial processes for the oxidation of liquid cycloparaffins are usually operated on such a large scale that there are significant incentives to increase the efficiency of the process, especially with respect to the unit cost, e.g., the cost per ton of product, for the manufacture of useful oxidation products. The percentage yield of useful products, per mole of cycloparaffin oxidized, and the amount of cycloparaffin oxidized, i.e., the conversion obtained as a result of one pass of cycloparaffin through the oxidation zone of the process, are both important process parameters. Such parameters have a major effect on the productivity of the process, i.e., the amount of useful oxidized products formed in the process in a given period of time, which is a prime factor in determining the unit cost for the manufacture of useful oxidation products. In addition, the composition of the oxidation products, e.g., the ratio of cycloalkanol to cycloalkanone, may be an important factor in the efficient operation of processes that convert the useful oxidized products into other products.
It has now been found that cycloparaffins may be catalytically oxidized to useful partial oxidation products, including a relatively high proportion of ketonic oxidation products, by using a process in which the oxidation catalyst is a combination of heavy metal compounds.